//改进前
setTimeout(function () {
    var a = 'hello'
    setTimeout(function () {
        var b = 'lagou'
        setTimeout(function () {
            var c = 'I ♥ U'
            console.log(a + b + c)
        }, 10)
    }, 10)
}, 10)

//改进后

var a = 'hello'
Promise.resolve(a).then((res) => {
    var b = 'lagou'
    return res + b
}).then((res) => {
    var c = 'I ♥ U'
    console.log(res + c)
})